Motor vehicles with a battery electric drive or a hybrid drive are becoming increasingly widespread. With such vehicles the drive energy for the electric drive is at least partly provided by one or a plurality of traction batteries. Such a traction battery can in particular be a lithium ion battery. If a lithium ion battery is discharged to below a discharge end voltage or is discharged further than corresponds to a critical state of charge, then there is a risk that the battery will be irreversibly damaged, e.g. by the chemicals contained in the battery cells destroying the battery itself. This process is irreversible and cannot be undone by re-charging the battery. Such deep discharging therefore in general necessitates replacement of the battery, which is associated with considerable cost. In addition, a battery electric vehicle or hybrid vehicle whose traction battery has been deeply discharged can no longer be readily put into operation. A battery electric vehicle with a deeply discharged traction battery must generally be towed. It is therefore known to monitor the state of charge of a traction battery of a motor vehicle in order to be able to prevent deep discharging.
According to WO 2012/085615 A1, an energy supply system of a motor vehicle comprises a battery and a battery control unit that monitors the state of charge of the battery. While the motor vehicle is not being operated, a state of charge of the battery is monitored by the battery control unit and if a lower threshold value of the state of charge is reached a communications device of the motor vehicle is activated in order to send an alarm to a receiving device located outside of the motor vehicle because of the critical state of charge. Recharging of the battery can then be initiated because of the alarm signal received by the receiving device.
It is known from EP 2 068 161 A2 to transmit data from a battery monitoring system installed in the motor vehicle to an external display device together with other diagnostic information about the motor vehicle.
In GB 2 455 160 A, a monitoring system for a battery module of an electric vehicle is disclosed wherein the physical and electrical variables of the battery module are recorded and transferred to an external system via a communications unit. If the received data contains an abnormal state of the battery module, the external system sends an error message to a user via wireless telecommunications means to remind the user that a service is due or to inform the user about the state of the motor vehicle.
The known methods and systems for monitoring a traction battery of a motor vehicle are not sufficiently reliable under all operating conditions. In particular, the output of an alarm signal relies both on a function of a monitoring system associated with the motor vehicle and also on the function of a wireless communications link. However, both cannot always be reliably guaranteed in such cases in which the critical state of charge of the traction battery can occur.